Embryological Heart Development

• Module 1 covers cardiovascular development

Embryological Heart Development

• Mostly derived from the mesoderm
• This is the first major system to function
• Appears in the 3rd week (18-20 days)
• Heartbeat begins during the 4th week (22-23 days)
• Blood is circulating during the 4th week

Heart Tube

• Cardiogenic area in the head region
• Primitive blood vessels found throughout
• Fusion into primitive heart tube occurs around 21 Days
• Primitive atrium, ventricle, bulbus cordis and truncus arteriosus present at 22 days

Cardiac Looping

• Primitive atrium, ventricle, bulbus cordis and truncus arteriosus present at 23 days
• Atrium & Sinus venosus present at 24 days
• Looping ends around day 28
• Aortic arch arteries, Truncus arteriosus, Left atrium and Ventricle present at 35 days

Septation

• This begins at the end of the 4th week
• Endocardial cushions begin separating the heart into right and left, upper and lower chambers
• These chambers become the atria and ventricles
• Completion occurs by the end of development

Atrial Septation

• The septum primum reduces blood flow from the right atrium to the left
• The opening between the right and left is called the ostium primum
• The septum continues growing until the ostium primum is nearly closed, decreasing blood flow
• Apoptosis in the septum primum causes an opening: the ostium secundum
• The ostium secundum allows for continued delivery of oxygenated blood to the left atrium
• The septum secundum nearly overlaps the ostium secundum, creating the foramen ovale
• This establishes a one-way passage for blood from the right atrium to the left

Atrioventricular Septum

• During the 4th week, the endocardial cushions grow toward each other
• This divides the heart into left and right atrioventricular canals
• Tissue along the periphery of each AV canal thins out to form AV valves

Truncus

• Swellings appear in the truncus arteriosus during the 5th week
• Those swellings start to migrate and rotate around each other
• This forms the aorticopulmonary septum
• The septum divides the truncus into the aortic and pulmonary channels
• These become the aorta and pulmonary trunk
• Other truncal swellings thin out to form the semilunar valves

Ventricle Septation

• Around the 4th week, muscular ventricle walls start expanding
• Medial ventricle walls grow toward each other and eventually merge, forming the interventricular septum
• The superior membranous portion of the interventricular septum from the endocardial cushions completes the closure of the interventricular foramen

Blood Flow Through Heart

• Prior to septation of the heart and truncus arteriosus, all blood from L and R atria goes from primitive ventricle through foramen to bulbous cordis and to truncus arteriosus
• This develops pulmonary arteries and aorta
• Can be defined as a "Double inlet left ventricle and double outlet right ventricle"
• After fusion of endocardial cushions and septation of truncus arteriosus into primitive pulmonary artery and aorta, the heart starts functioning as 4 chambered
• Looping is completed and great arteries separate, as well as valves and their respective atria migrate and develop
• A final form is achieved at ~8 weeks

Embryonic Blood Flow

• Vitelline vein
• Umbilical vein
• Heart tube

Blood Flow - Umbilical Arteries & Vein

• The umbilical cord is formed by week 5
• Protects the vessels that travel between the fetus and the placenta
• The umbilical cord normally contains two umbilical arteries and one umbilical vein
• These are embedded within a loose, proteoglycan-rich matrix called Wharton's jelly

Fetal Blood Circulatory System

• The placenta has 2 separate circulatory systems: Maternal-placental (uteroplacental) and Fetal-placental (fetoplacental) circulations
• All nutrition and O₂ comes from mother to fetus through the umbilical vein
• Nutrient depleted blood, waste products and carbon dioxide from the fetus goes back to the mother through umbilical arteries
• Fetal circulation has 3 shunts to bypass liver and lungs
• Foramen Ovale moves blood from the Right atrium to the Left atrium, bypassing the right ventricle
• Ductus arteriosus moves blood from the pulmonary artery to aorta
• Ductus venosus moves blood from the umbilical vein to IVC and Right atrium

Circulation After Birth

• After birth, the umbilical cord (blood & O₂ supply) is clamped and then cut
• The lungs must start oxygenating the blood asap
• The first breath brings O₂ in the lungs and action of breathing
• Clamping the cord improves blood flow to the lungs
• Increased systemic vascular resistance increases the pressure in the Left atrium (increased amount of blood from pulmonary veins dumps in Left atrium)
• Left atrial pressure becomes higher than the Right atrial pressure (Right side dumps into air-filled lungs)
• High Left arterial pressure pressing against the septum primum, closing the foramen ovale
• The smooth muscle in the ductus arteriosus constricts in the presence of high O₂ levels
• Functionally completes the separation of the heart into two pumps--right and left sides of the heart

Newborn Circulatory Milestones

• The foramen ovale closes and can no longer act as a shunt between the two atria of the heart
  • Functionally closes immediately after birth
  • Anatomically closes within 12 months
• The ductus arteriosus closes and no longer provides a communication between the pulmonary artery and aorta
  • Functionally closes 12-24hrs
  • Anatomically closes in 2-3 weeks
• The ductus venosus constricts so that all blood entering the liver passes through the hepatic sinusoids
  • Functionally closes immediately after birth
  • Anatomically closes after 1-3 months (ligamentum venosum)
• Occlusion of the placental circulation causes an immediate drop in blood pressure
  • Systolic range 46 to 94mmHg, diastolic range 24 to 57mmHg

Abnormalities of Looping

• Associated with isomeric cardiac lesions
• Dextrocardia: heart loops to left instead of to right
• Situs solitus, situs inversus or situs ambiguus

Dextrocardia FYI

• Situs solitus: ~90% have significant cardiac issues
  • May have aorta connecting to right ventricle (Double Outlet Right Ventricle)
  • Usually with Ventricular Septal Defect (VSD)
  • May have Endocardial Cushion Defects (abnormal or absent walls)
  • May have Pulmonary Valve Stenosis
• Situs inversus: ~5-10% have significant cardiac issues
• Situs ambiguus: severe disorganization of organs with high mortality rate

Endocardial cushion/septal defects

• Aka atrioventricular (AV) canal or septal defects
  • Atrial septal defects
  • Ventricular septal defects
  • Defects of the great vessels
    • Tetralogy of Fallot
    • Transposition of the Great Vessels

Circulatory System Defects

• Small % of umbilical cords contain only one artery
  • Associated with chromosomal abnormalities and congenital malformations (neural tube and cardiac defects)
• Arterial system defects
  • Patent ductus arteriosus
  • Coarctation of the aorta
  • Double aortic arch (vascular ring)
• Venous system defects
  • Double inferior or superior vena cava
  • Absence of the inferior or superior vena cava
• Patent Foramen Ovale (PFO)